1. Field of the Invention
This invention relates to an assembly having plural environmental seals for sealing an active matrix liquid crystal display module.
2. Description of Related Art
High resolution, high speed liquid crystal displays for depicting graphical or linguistic information comprise many pixels, each of which may be driven or controlled by an active matrix element such as a transistor. Illustrative publications relating to such displays include Edmonds, U.S. Pat. No. 3,832,034 (1974); Hamaguchi, U.S. Pat. No. 4,838,654 (1989); Bowman et la., U.S. Pat. No. 4,910,579 (1990); Shannon, U.S. Pat. No. 4,931,787 (1990); Te Velde, U.S. Pat. No. 5,005,951 (1991); Flasck, U.S. Pat. No. 5,022,750 (1991); Flasck, U.S. Pat. No. 5,024,524 (1991); Flasck, U.S. Pat. No. 5,108,172 (1992); Kamath et al., U.S. Pat. No. 5,233,445 (1993); IBM, EP 0,112,417 (1984); N. V. Philips, EP 0,294,898 (1988); Garwin, IBM Technical Disclosure Bulletin, Vol. 22, no. 8A, pp. 3447-8 (January. 1980); and Huntley, IBM Technical Disclosure Bulletin, Vol. 23, no. 1, pp. 347-8 (June. 1980).
The active matrix elements are disposed in an array, each element helping define a pixel and controlling the visual state of liquid crystal material associated therewith. Depending on the desired switching state of an active matrix element, a voltage (electric field) is applied or not to the liquid crystal material. In turn, the liquid crystal material is switched from one electro-optical state to another. For example, a pixel in the "field-on" state may permit incident light to be transmitted through the liquid crystal material and to be specularly reflected by a reflector positioned behind it, back towards the incidence side (albeit angularly displaced by operation of the laws of reflection), while a pixel in the "field-off" state may prevent such reflection by scattering or absorbing the light. The combination of many "on" and "off" pixels generates an image which can be viewed directly, or projected onto a screen for viewing, or viewed as a virtual image with the aid of appropriate optics. By combining red, green and blue images, either via sequential illumination with red, green and blue light or via dedicated red, green and blue pixels, a colored image can be formed.
One type of liquid crystal material that may be used is a liquid crystal composite comprising plural volumes or droplets of liquid crystals are contained within a polymer matrix, such as disclosed in Fergason, U.S. Pat. No. 4,437,047 (1984).
The operation of such displays generates heat which must be dissipated. The heat can arise from the absorption of light incident from a light source (which is especially intense for projection displays) and/or electrical power consumption by the active matrix elements. A conventional way to dissipate heat is to adhesively bond the backside of a substrate containing the active matrix elements to a heat sink, such as a block of metal. However, as the local temperature fluctuates during the course of operation of the display, thermally induced mechanical stresses can develop in the substrate due to differences between its and the heat sink's coefficients of thermal expansion. The stresses can propagate to the liquid crystal material itself, which is confined between the backside substrate and a front-side substrate. Liquid crystal materials may be sensitive to such stresses and may have their electro-optical properties adversely affected. A solution to this problem is disclosed in copending, commonly assigned U.S. patent application of Gray et al., Ser. No. 08/713,909, filed Sep. 13, 1996 (hereinafter the "Gray application"), the disclosure of which is incorporated herein by reference. The Gray application discloses an assembly for dissipating heat from an active matrix liquid crystal display module while avoiding developing thermally induced mechanical stresses. The module is affixed to a circuit board containing electrical leads for connection to drive electronics, in a way such that the substrate containing the active matrix elements is suspended through a window or through-hole in the circuit board and is sunken or nested into a soft elastomeric gel which is electrically non-conductive but thermally conductive. The gel provides the requisite thermal conductivity to dissipate the heat via a heat sink with which the gel is in thermal contact.
Another important performance parameter is effectiveness of the seal isolating the liquid crystal material from the external environment. In a typical design, the liquid crystal material is sandwiched between a top and a bottom substrate, such as glass, polymer or a semiconductor. The edges of the sandwich are sealed by an edge sealant. The assembly in the Gray application follows this design. However, it is possible for environmental agents--especially moisture--to find their way across the edge sealant, contaminating the liquid crystal material and adversely impacting its electro-optical properties. By way of a specific illustration, for liquid crystal composites, the voltage holding ratio (VHR) is lowered if moisture ingress occurs. The liquid crystal material acts as a capacitor, storing charge for approximately 16 msec between frame refreshes when the pixel capacitors are re-charged. A reduced VHR results in increased charge leakage across the liquid crystal material between frame refreshes, thereby reducing the amount of light transmitted between frame refreshes.